Piezoelectric device and method for using same

Abstract

A piezoelectric device, which has bipolar polarization-electric field (Pr-E) hysteresis characteristics of a piezoelectric material asymmetrically biased, when a first and second coercive electric fields respectively having smaller and larger absolute values are defined as Ec1 and Ec2 and a bias ratio of the coercive electric field is defined as [(Ec2+Ec1)/(Ec2−Ec1)]×100[%], includes a piezoelectric element unit including a piezoelectric body film whose bias ratio is 20% or more, the piezoelectric element unit operating with an electric field intensity smaller than that of the first coercive electric field. The piezoelectric device includes a refresh voltage applying circuit configured to apply a voltage to maintain operation performance of the relevant device, the voltage having an electric field intensity larger than the electric field intensity for operating the device and being equal to or less than three times |Ec1|, such that a polarized state of the piezoelectric body film is restored.

What is claimed is: 
     
       1. A piezoelectric device operating by a piezoelectric effect and an inverse piezoelectric effect of a piezoelectric body film, the device comprising:
 a piezoelectric element unit; and 
 a refresh voltage applying circuit, wherein 
 bipolar polarization-electric field (Pr-E) hysteresis characteristics of a piezoelectric material are asymmetrically biased with respect to an axis of a zero electric field, and 
 when a first coercive electric field having smaller absolute value is defined as Eel and a second coercive electric field having larger absolute value is defined as Ec2 of coercive electric fields in the piezoelectric material, and 
 a bias ratio of the coercive electric field is defined as [(Ec2+Ec1)/(Ec2−Ec1)]×100[%], 
 the piezoelectric element unit includes the piezoelectric body film whose bias ratio of the coercive electric field is equal to or more than 20%, and the piezoelectric element unit operates with an electric field intensity smaller than that of the first coercive electric field, and 
 the refresh voltage applying circuit applies a voltage in order to restore a polarized state of the piezoelectric body film and maintain operation performance of the piezoelectric device, the voltage having an electric field intensity larger than the electric field intensity for the operating and being equal to or less than three times the absolute value |Ec1| of the first coercive electric field, wherein 
 the piezoelectric element unit includes a piezoelectric element for drive configured to operate by an inverse piezoelectric effect, and a piezoelectric element for detection configured to operate by a piezoelectric effect, 
 the piezoelectric device includes a drive circuit configured to supply a drive voltage to the piezoelectric element for drive, and a detection circuit configured to detect a voltage signal from the piezoelectric element for detection, 
 the refresh voltage applying circuit is installed in at least one circuit of the drive circuit and the detection circuit, and 
 the piezoelectric device further comprises a control unit configured to apply a voltage to the piezoelectric element unit from the refresh voltage applying circuit in a case where a detection voltage is lower than a reference value, the detection voltage being output from the piezoelectric element for detection when the piezoelectric element for drive is driven with a predetermined drive voltage. 
 
     
     
       2. The piezoelectric device according to  claim 1 , wherein
 the bias ratio of the piezoelectric body film is equal to or more than 70%. 
 
     
     
       3. The piezoelectric device according to  claim 1 , wherein
 an electronic circuit unit including at least one circuit of the drive circuit and the detection circuit, and the refresh voltage applying circuit is constituted by an integrated circuit. 
 
     
     
       4. The piezoelectric device according to  claim 1 , wherein
 a magnitude of a voltage supplied from the refresh voltage applying circuit to the piezoelectric element unit is equal to or less than 5V. 
 
     
     
       5. The piezoelectric device according to  claim 1 , wherein
 the piezoelectric element unit has a laminated structure in which a first electrode, the piezoelectric body film, and a second electrode are laminated, and 
 the refresh voltage applying circuit applies a minus voltage to the second electrode when the first electrode is grounded. 
 
     
     
       6. The piezoelectric device according to  claim 1 , wherein
 the piezoelectric body film is perovskite-type oxide. 
 
     
     
       7. The piezoelectric device according to  claim 1 , wherein
 the piezoelectric body film is formed of one kind or plural kinds of perovskite-type oxide expressed by a general formula (P-1) below (which may contain inevitable impurities),
   Pb a (Zr b1 Ti b2 X b3 )O 3   general formula (P-1)
 
 
 (in the formula (P-1), X is at least one kind of metal element selected from element groups of a V group and a VI group; a>0, b1>0, b2>0, b3≧0; while a≧1.0 and b1+b2+b3=1.0 as a standard, these numerical values may include a tolerance from 1.0 in a range where a perovskite structure can be taken). 
 
     
     
       8. The piezoelectric device according to  claim 7 , wherein
 X of the piezoelectric body film is Nb, b3 is equal to or more than 0.05 and equal to or less than 0.3. 
 
     
     
       9. The piezoelectric device according to  claim 7 , wherein
 X of the piezoelectric body film is at least one kind of metal element of Nb and Bi. 
 
     
     
       10. The piezoelectric device according to  claim 7 , wherein
 a is equal to or more than 1.1 in the general formula (P-1) for the piezoelectric body film. 
 
     
     
       11. A method for using a piezoelectric device which is operated by use of at least one of a piezoelectric effect and an inverse piezoelectric effect of a piezoelectric body film and with an electric field intensity smaller than that of a first coercive electric field, the piezoelectric device including a piezoelectric element for drive configured to operate by an inverse piezoelectric effect, and a piezoelectric element for detection configured to operate by a piezoelectric effect, wherein
 bipolar polarization-electric field (Pr-E) hysteresis characteristics of a piezoelectric material are asymmetrically biased with respect to an axis of a zero electric field, and 
 when a first coercive electric field having smaller absolute value is defined as Ec1 and a second coercive electric field having larger absolute value is defined as Ec2 of coercive electric fields in the piezoelectric material, and 
 a bias ratio of the coercive electric field is defined as [(Ec2+Ec1)/(Ec2−Ec1)]×100 [%], 
 the piezoelectric body film whose bias ratio of the coercive electric field is equal to or more than 20% is used, and 
 in a case where a detection voltage is lower than a reference value, the detection voltage being output from the piezoelectric element for detection when the piezoelectric element for drive is driven with a predetermined drive voltage, a voltage is applied in order to maintain operation performance of the piezoelectric device, the voltage having an electric field intensity larger than the electric field intensity for the operating and being equal to or less than three times an absolute value |Ec1| of the first coercive electric field, such that a polarized state of the piezoelectric body film is restored.